Hi,
I'm sorry to say that your objection to heaters inside a filter is unreasonable. I mean, in the first place, the whole point of having a filter is to grow bacteria, so heat causing more bacteria to grow is a good thing. The waste that you are heating is also the food that the bacteria feed on and if the heat helps to accelerate the breakdown of this waste then this is also a good thing. So really your arguments are actually excellent reasons for an in filter heater. Furthermore, the only way you can get more dirt in your filter is by sucking up more dirt from the tank, so more dirt in the filter is a good thing. One needs to clean the filter regularly to avoid problems, but that's standard practice.
Cheers,